sunrpc: simplify xdr_partial_copy_from_skb

csum_partial_copy_to_xdr can handle a checksumming and non-checksumming
case and implements this using a callback, which leads to a lot of
boilerplate code and indirect calls in the fast path.

Switch to storing a need_checksum flag in struct xdr_skb_reader instead
to remove the indirect call and simplify the code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>